In composite video signals, the luminance and chrominance signals are transmitted in the same bandwidth. The chrominance signal is modulated onto a sub-carrier signal and then added to a luminance signal to create a baseband signal. The baseband signal is then modulated onto a carrier for transmission. At the decoder, the baseband signal must be processed to separate the chrominance and luminance signals. Some mixing of the two signals is unavoidable, resulting in YC-interference noise, also called cross-color and cross-luminance. Cross-color is the result of decoding part of the luminance signal as color data. The changes in the sub-carrier phase cause the YC-interference noise to affect the decoded digital signal in a predictable way. For still pixels (pixels whose ideal, noise-free component values are not changing between frames), the value of the YC-interference noise in the decoded cross-luminance is the result of decoding part of the color signal as luminance data.
For both National Television System Committee (NTSC) and phase alternating line (PAL) transmission standards, the phase of the color sub-carrier changes with each frame in a specified pattern. For NTSC, the phase of the color sub-carrier inverts every frame. As a result, in unchanging “still” pixel locations, YC-interference noise causes the components of the pixel location to oscillate about an ideal value at the frame rate. Such an oscillating effect which is often referred to as shimmering is illustrated by the following equations:yF—in[T]=yid+yc yF—in[T−2]=yid−yc yF—in[T−4]=yid+yc crF—in[T]=crid+cry crF—in[T−2]=crid−cry crF—in[T−4]=crid+cry cbF—in[T]=cbid+cby cbF—in[T−2]=cbid−cby cbF—in[T−4]=cbid+cby where                Yid, crid, cbid—Ideal value of components of the pixel location        Yc, cry, cby—Absolute value of YC-interference noise        F_in[t]—The Sequence of input video fields        T—The index of the most recent input field        
The ideal value of still pixel locations can be recovered by averaging the pixel values in adjacent fields. FIGS. 1 and 2 illustrate the pattern of the color sub-carrier phase change in NTSC video and the resulting pattern in the YC-interference noise at still pixel locations.
For PAL, the phase of the color sub-carrier shifts by 90° every frame so that the sub-carrier inverts every 2 frames. YC-interference noise at unchanging pixel locations causes the actual value of the pixel to oscillate around the ideal value at half the frame rate, as shown by the following equations:yF—in[T]=yid+yc yF—in[T−4]=yid−yc yF—in[T−8]=yid+yc crF—in[T]=crid+cry crF—in[T−4]=crid−cry crF—in[T−8]=crid+cry cbF—in[T]=cbid+cby cbF—in[T−4]=cbid−cby cbF—in[T−8]=cbid+cby 
The ideal value of still pixel locations can be recovered by averaging the pixel values in fields T and T−4. FIGS. 3 and 4 illustrate the pattern of the color sub-carrier phase changes in PAL video and the resulting pattern in the YC-interference noise at still pixel locations.
The effect of the changing phase of the color sub-carrier on the YC-interference noise makes it possible to use temporal filtering to eliminate the cross-luminance and cross-color in the separated luminance and chrominance signals. Because non-adaptive temporal filtering causes visual artifacts in moving areas of the video, some method of adapting the filter to prevent those artifacts is needed. One conventional approach uses a motion detector to switch the filter off in areas of the video that have motion. Such a conventional approach is shown in FIG. 5. The problem with such an approach is that the YC-interference noise that the filter is trying to eliminate can cause motion detectors to incorrectly label still areas as motion areas.
It would be desirable to implement a detector that identifies and filters particular pixels whose components exhibit the oscillation pattern caused by YC-interference noise.